Sam,
This is a tough one because we know our audio memory is a short one. What I do remember is there was what appeared to be more detail. I believe the high end was a bit more defined and had a bit more clarity and the the mids were even more sweet. The bass, particularly the upper bass, was more refined and more distinguishable. I think this would be easier to demonstrate with a double bass, aka stand up bass for the upper bass range.
I think all of this was was due to the change between the 17LS and the 17LS2 where they upgraded/changed the caps to the "famous" CJ (custom) caps. I also believe they "beefed up" the power supply. What this end result is what called a quieter background or it is sometimes referred to as blacker blacks. I think in another type of analogy, this demonstrates is it just has more dynamic clarity with less noise throughout the audio spectrum.
Now this is not to say the the 17LS is bad, which it is not at all. As you well know and are experiencing now, the great sound of a CJ preamp as well as the amp. These two do make a really sweet combination.
I only mentioned that I am selling my 17LS2 just to pull at you a bit. I know you just got the 17LS and it is a great preamp and you will have many years of enjoyment, not to try and sell you mine.
Remember the most important thing is not to get wrapped around all the specs and "me too" but to sit back, relax and enjoy the music.
Jeff